塔里木盆地东北部奥陶-志留系沉积成岩作用

Ordovician-Silurian sedimentation and diagenesis in northeastern Tarim basin

郑冰 承秋泉 高仁祥

Zheng Bing Cheng Qiuquan Gao Renxiang

塔里木盆地东北地区奥陶系为一套粉砂质泥岩和灰岩;志留系砂砾岩构成4个沉积旋回,为典型的复理石建造,含油气层位于旋回的底部,4个旋回均已倒转。成岩作用在埋深1500 m已很显著,富含大气CO水中沉淀亮晶方解石(Ⅰ),δc为-4‰-6‰,δ0为-5‰～-6‰。埋深2400-2600 m,硫酸盐还原带形成的碳酸盐(Ⅱ)δC为-7‰～-10‰,δO为-8‰～-10‰,长石溶解形成第一次生孔隙带。埋深3800-4000 m,甲烷生成带脱羧作用产生的CH形成波状消光白云石 (Ⅲ),δC为-1‰～-16‰,而CO直接形成的碳酸盐δC为20‰右右,δO均在-12‰～-13‰,SiO次生包体测温110-120℃,形成第二次生孔隙带。埋深4500-4700 m为湿气生成带,形成的碳酸盐(Ⅳ)δC为-27‰～-31‰,δO为-13‰～-14‰,包体测温130-140℃,含铁方解石大量溶解形成第三次生孔隙带。埋深达到5000 m以上时产生大量干气(Ⅴ), δC为-18‰～-20‰,δO为-1‰～-14‰,与包体均一温度150-155℃相一致,形成第四次生孔隙带,孔隙度仅15%-20%。海西运动期间褶皱地层可能倒转,抬升剥蚀可能达5000 m左右,接着发生"退成岩作用"。

The Ordovician is composed of silty mudstone and limestone, and the Silurian is a typical flysch formation composed of four depositional cycles of glutinite in the northeastern Tarim basin. The reservoirs occur at the bottom of each cycle. All of the 4 cycles have been inverted. Diagenesis is obvious at a depth of 1 500 m where sparry calcite (Ⅰ) has been precipitated from water rich in atmospheric CO with δC in the range of -4‰ and -6‰, and 818O in the range of -5‰ and -6‰. At a burial depth of 2 400-2 600 m, δC and δO in the carbonate (Ⅱ) formed in the sulfate reduction zone are in the range -7‰ to -10‰ and -8‰ to -10‰, respectively, where the dissolution of feldspar constitutes the 1st secondary pore zone. Dolomites (Ⅲ) with wavy extinction have been formed at a burial depth of 3 800-4 000 m, along with the decarboxylation in the methane generating zone, the δC is in the range of -11‰ and -16‰, while in the carbonate directly formed from CO, the δC is ±20‰, and the δO both in dolomite and carbonate range from -12‰ to -13‰, temperatures measured from secondary SiO inclusions are in the range of 110℃ and 120℃. This interval constitutes the 2nd secondary pore zone. The depth interval of 4 500-4 700 m is a wet gas generating zone, the δC and δO in carbonate (Ⅳ) formed in this interval are in the range of -27‰ to -31‰ and -13‰ to -14‰, respectively, and the temperature measured from inclusion thermometry is 130 -140℃; and the massive dissolution of ferroan calcite has resulted in the formation of the 3rd secondary pore zone. Large amount of dry gas (Ⅴ) were generated at a burial depth of over 5 000 m, where the 8 C ranges from -18‰ to -20‰ and δO ranges from -13‰% to -14‰, being accordant with the inclusion's homogenization temperature of 150-155℃. This interval constitutes the 4 th secondary pore zone with porosity in the range of 15% to 20%. The folded strata might have been inverted during Hercynian movement, uplifted and eroded about 5 000 m and associated with'retrodiagenesis'.

10.11743/ogg20060114